Coating



United States Patent COATING Charles A. Baer, Needham, Wallace F. Bugbee, Boston, and Philip J. 'Clough, Reading, Mass, assignors to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Application May 10, 1957, Serial No. 658,919

9 Claims. (Cl. 1486.35)

This invention relates to coating and more particularly to coating of black iron for the purpose of producing a tin plate substitute.

This application is in part a continuation of application Serial No. 423,270, filed April 15,

been a lack of appreciation of the fundamentalrequirements for an adherent, corrosion-resistant coating of aluminum on black iron.

Accordingly, it is a principal object of the present invention to provide an adherent, corrosion-resistant coating of aluminum on black iron.

Another object of the invention is to provide such a coating which is extremely thin, on the order of a few micro-inches in thickness.

Still another object is to provide such a coating which is applied to the black iron with vapor deposition tech niques.

Still another object of the invention is to provide an improved article of manufacture.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing which is a graph showing the time temperature relationships forming a part of the present invention.

In a preferred embodiment of the present invention, black iron plate, after thorough cleaning, is passed through a vacuum chamber where a low pressure (on the order of 10 microns Hg abs. or less) is maintained. During passage through this chamber, the black iron is subjected to an aluminum vapor stream from a suitable source of aluminum vapors, such as shown in US. Patents 2,643,- 201 and 2,665,223. As described more fully hereinafter, the aluminum film deposited during the coating operation is not particularly adherent and certain precautions are taken in its manufacture to provide a rather porous film. The black iron having the porous aluminum coating is then passed out of the coating chamber and is given an aftertreatment comprising oxidation of the iron-aluminum interface at a temperature which is preferably maintained below the annealing temperature of the black iron. In a preferred embodiment of the invention, the aluminumcoated black iron is heated in an oxidizing atmosphere to a temperature above about 350 F. and below the annealing temperature of black iron (about 700 F.). At the upper temperature limits, the temperature need be maintained for only a few seconds, while at the lower temper'ature limits, particularly below 500 F., the tempera- 2,887,419 Patented May 19, 1959 ture must be maintained for a time on the order of minutes rather than seconds. The product is maintained at this temperature for a time sufficiently lon so that oxidation of the aluminum and iron at the iron alurninum interface forms an adherent bond between the aluminum coating and the black iron plate. While the exact nature of this bond is not completely understood, it is believed to be an iron aluminate bond. t p

The drawing illus'u'atesa graph which shows the relationship of time of heating to temperature of heating to obtain a satisfactory bond by oxidation. In this graph, the line A--B represents thevery minimum tirne-at-temperature curve. Almost without exception, tests run below this curve A-B have failed togive a satisfactory bond. Tests run between curves. A-B and C--D have given sporadically successful bonds, while tests run above curve C-D have been almost completely satisfactory.

It has 'alsobeen discovered that the. ability to form the adherent iron aluminate bond by oxidation after coating is a function of the coating conditions as wellas the oxidation conditions. There are three principal factors to be considered in the formation of the initial aluminum coating. .These are (a) the iron temperature during coating, (b) the temperature of the source of aluminum vapors, and (c) the distance traveledby the aluminum vaporsfrom the source to the substrate. When the aluminum film is formed by depositing aluminum vapors which have traveled more than one mean free path from the aluminum source to the substrate, the resultant film is readily bonded withthe base iron by subsequent oxidation treatment. On the other hand, an aluminum coating formed under conditions which permit deposition of aluminum vapors which have traveled less than one mean free path has been extremely difficult to bond by subsequent oxidation treatment. This is particularly true when the operation is carried out at near the. limits for the other conditions specified above. For example, when the substrate is hot (e.g., near 200 F.) or the source is hot (e.g., 1600 C.), a coating applied at less than one mean free path can not be made adherent by subsequent oxidation treatment in any reasonable length of heating time. The mean free path is the average distance which a molecule of gas travels before striking another molecule. It is given by the following equation:

n is the number of molecules per cubic em. 8 is the eifective molecular diameter The value for 8 for aluminum given in the Handbook of Chemistry and Physics (31st edition, Chemical Rub- 'ber Publishing Company) is 2.86 10- cm. Using this value and the above equation at 1 micron Hg abs., \=7.7 cm.=3.03 inches. Since'the mean free path is inversely proportional to the pressure at low pressures, the following values can be calculated:

Mean Free Path P (microns Hg abs.)

Inches Om.

It is believed that the dlfl'lbl llty in obtaining a good bond by subsequent oxidation when the coating takes place at less than" the-meanfreepath is'due tothe-fact that the higher energy aluminum atoms encountered under these conditionsform a denser coating. Thisdenser coating is only slowlypenetra'ted by the oxygen is necessary to form"'theadherent'hondi" This) general-prebiem arises if the temperament theblack irerr is" i'r'i'a tai'n'ed above about 1 5' F. dining-the coating operation. This isparticularly true when-thealurnihuhi vapor source has a high temperature on the order of 1600 C. In this case, the'aluminum deposited onthe he'ated substrate does not have sutficient adhesion by itselfand it's adhesion is difiicult to improve by subsequent" oxidat" eheat treatment. At lowalumin warp eurcerernperatures(e.g.; 1200 CL), the substra e can Be heated t'da temperature as highas 3'00 Rwiiihou't' destroying the hardeningeffect of the'later oxidation step. Accordingly, in a preferred form of the invention, the coating" is carried out under such as l200 C. or'helow.

Example I A sheet of black iron 0.0095 incli thick was thoroughly cleaned and positioned in a vacuum chamber. The chainher was evacuated to a ressure-onthe order of.5 micron Hg'abs: An aluminunr-carryingfilamerit spaced 1 1 inches from the black iron sheet" was heate'd'to a temperature of 1150 to [200 C. to evaporate the aluminum. A coating of T6 microinches thick (as measured by electrical resistance) was deposited on the black iren, while the black iron was maintained at room temperature. 7 The coated hl'ack iron was then removed from the coating chamber and the adhesion of the coating was" tested by means of a pressure-sensitive adhesive tape and was fdund to be very poor. The" coated black" iron was then heated in air to600' F. for 1 5sec'ond's'. adhesion was tested again and found to be excellent.

Example ll A sheet of Black iron was treated as in Example I except that the coated blaek" iron washeated in aif'to500 F, for 1 minute. An excellent hondwa's for-med between the aluminum andiron:

Exam le III A sheet of black iron was treated as in Example I except that the coated black iron was heated at 500 F. in

Example V A sheet of black iron was" treated as in Example I except that the black iron was heated to 200 F. during coating. In this case, thealuminum vapor source was at about .1200" C. The adhesion of the aluminum coating was poor as removed firom the coating chamber but became excellent after heating in air to 600 F. for 5 seconds.

Example VI A sheet of black iron was treated as in- Example I- except that the black iron was only 6 inches away from the source of aluminum vspers. Since the pressure was .4 micron Hg abs, the'hlack iron was less than one mean 4 free path from the source. The coating had only fair adhesion and was not appreciably improved by heating to .500" F. for 1 minute. After 3 minutes heating at 500 F., the adhesion was satisfactory.

In the specification and claims, reference is made to the thickness of the aluminum film deposited on the black iron. Due to the extreme thinness of this film, its actual thickness cannot be readily measured. Accordingly, elect-rical r'esista'mce rheasurements'have been used to obtain calculated film thicknesses. The specific resistance of pure aluminum is' 218x10 ohms per centimeter" cube. thus, a centimeter square which is 0.1 rhicr on=4 micro inches=10- cmr thick will have a resistance of =0.28- ohmsper' square 1 microinchg 1.12 ohms per square 8 microinch'es g0. 143 ohm per'square 16 microinches007 ohm per square, etc.

It should he remembered that thickness calculated from resistance measurement represents a value. The actual thickness is undoubtedly greater in most cases since the calculation is based on two usually erroneous assumptions:

('1) That the film is completely continuous throughout its depth and is completely free of submicroscopic cracks so thatresistance is at a (2) That the film is packed to a density.

While the invention has been particularly described in connection with its use for a tin plate substitute, itcan be equally utilized for protecting ferrous objects in general.

Sincecertain changes may be madein' the above'process and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall-beintenpreted as illustrative and not in a limiting sense.-

What is claimed is:

1. The process of producing a shiny, adherent, corrosion-resistant aluminum coating on black iron which comprises the steps of exposing black iron to a source of aluminum vapors in a vacuum chamber maintained at a pressure on the order of 10 microns Hgabs. and less to deposit on the black iron a film of aluminum having a thickness between about 1 to 30 microinches, andthereafter heating the aluminum-coated black iron in an oxidizing atmosphere at a temperature above about 350 F. and below about 700 F., said heating being continued for at least a minimum- -time ranging between a few seconds at 700 F. to a few minutes at 350 F. and being sufiiciently long to forman iron aluminate bond between the "black iron and the thin aluminum. coating.

2. The process of producing a shiny, adherent, corrosion-resistant coating on black iron which comprises the steps of exposing black iron to a source of aluminum vapors in a vacuum chamber maintained at a-press'ure on the order of 10 microns Hg abs. and less to deposit on the iron a' film of aluminum having athickness between about 1 to 30 microinches, the iron being maintained more than one mean free path of an aluminum molecule away from the aluminum vapor sources during most of the time while it is exposed to the aluminumvapor source, and thereafter heating the aluminum-coated black iron in an oxidizing atmosphere at a temperature above about 350 F. and below about 700 F., said heating being continued for at least a minimum time ranging between a few seconds at 700 F. to a few minutes at 350 F. and being sufficiently long to form an adherent bond between the iron and the thin aluminum coating.

3. The process of claim 2 wherein the source of aluminum vapors is maintained at a temperature on the order of 1150 C. to 1200 C. p

4. The process of' claim 2' wherein the source of arurninuin vapors is maintained at atemperaturon the order of 1400 C. and above and the black iron is maintained at a temperature below 150 F. during coating.

5. The process of producing a shiny, adherent, corro sion resistant aluminum coating on black iron which comprises the steps of exposing black iron to a source of aluminum vapors in a vacuum chamber maintained at a pressure on the order of microns Hg abs. and less to deposit on the iron at film of aluminum having a thickness between about 1 to 30 microinches, the iron being maintained more than one mean free path of an aluminum molecule away from the aluminum vapor source during most of the time while it is exposed to the aluminum vapor source and the iron being maintained at a sufliciently low temperature so that the resultant aluminum coating is relatively non-adherent, and thereafter heating the aluminum-coated black iron in an oxidizing atmosphere at a temperature above about 350 F. and below about 700 F., said heating being continued for at least a minimum time ranging between a few seconds at 700 F. to a few minutes at 350 F. and being at least as long as the times indicated by the line C-D in the graph of the drawing.

6. The process of producing a tin plate substitute which comprises the steps of moving a strip of black iron past a source of aluminum vapors in a vacuum chamber maintained at a pressure on the order of 10 microns Hg abs. and less to deposit a thin, poorly adherent film of aluminum on the black iron, and thereafter heating the aluminum-coated black iron in an oxidizing atmosphere at a temperature above about 350 F. and below the annealing temperature of the black iron, the heating being continued for at least a minimum time ranging between a few seconds at temperatures as high as 500 F. and above to a few minutes at 350 F. and being sufiiciently long to form an adherent iron aluminate bond between the black iron and the thin aluminum coating.

7. The process of producing a shiny, adherent, corrosionresistant aluminum coating on black iron which comprises the steps of exposing black iron to a source of aluminum vapors in a vacuum chamber maintained at a pressure on the order of 5 microns Hg abs. and less to deposit on the iron a film of aluminum having a thickness between about 1 to 30 microinches, the iron being maintained more than one mean free path away of an aluminum molecule from the aluminum vapor source during most of the time while it is exposed to the aluminum vapor source, and thereafter heating the aluminumcoated black iron in an oxidizing atmosphere at a temperature above about 350 F. and below about 700 F., said heating being continued for at least a minimum time ranging between a few seconds at 700 F. to a few minutes at 350 F. and being suificiently long to form an adherent bond between the iron and the aluminum coating.

8. The process of producing a shiny, adherent, corrosion-resistant aluminum coating on black iron which comprises the steps of exposing black iron to a source of aluminum vapors in a vacuum chamber maintained at a pressure on the order of 1 micron Hg abs. and less to deposit on the iron a film of aluminum having a thickness between about 1 to 30 microindhes, the iron being maintained more than one mean free path away of an aluminum molecule from the aluminum vapor source during most of the time while it is exposed to the aluminum vapor source, and thereafter heating the aluminumcoatcd black iron in an oxidizing atmosphere at a temperature above about 350 F. and below about 700 F., said heating being continued for at least a minimum time ranging between a few seconds at 700 F. to a few minutes at 350 F. and being sufficiently long :to form an adherent bond between the iron and the thin aluminum coating.

9. A composite article of manufacture comprising an unannealed black iron sheet having a vacuum-deposited coating of aluminum thereon, the aluminum coating having a thickness on the order of 1 to 30 microinches, and forming a corrosion-resistant coating on said black iron which is superior to quarter-pound tin plate, the interface between the aluminum coating and the black iron having been oxidized by heating to a temperature above 350 F. and below the annealing temperature in an oxidizing atmosphere to form an adherent iron aluminate bond therebetween.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE PROCESS OF PRODUCING A SHINY, ADHERENT, CORROSION-RESISTANT ALUMINUM COATING ON BLACK IRON WHICH COMPRISES THE STEPS OF EXPOSING BLACK IRON TO A SOURCE OF ALUMINUM VAPORS IN A VACUUM CHAMBER MAINTAINED AT A PRESSURE ON THE ORDER OF 10 MICRONS HG ABS, AND LESS TO DEPOSIT ON THE BLACK IRON A FILM OF ALUMINUM HAVING A THICKNESS BETWEEN ABOUT 1 TO 30 MICROINCHES, AND THEREAFTER HEATING THE ALUMINUM-COATED BLACK IRON IN AN OXIDIZING ATMOSPHERE AT A TEMPERATURE ABOVE ABOUT 350*F. AND BELOW ABOUT 700*F., SAID HEATING BEING CONTINUED FOR AT LEAST A MINIMUM TIME RANGING BETWEEN A FEW SECONDS AT 700*F. TO A FEW MINUTES AT 350*F. AND BEING SUFFICIENTLY LONG TO FORM AN IRON ALUMINATE BOND BETWEEN THE BLACK IRON AND THE THIN ALUMINUM COATING. 